Do some searching for the specific pickup dimensions. For example, a quick search on '19 Dodges showed that a 1500 has a 140.5" wheelbase while the 2500 is 149.3. The bed length was the same but the wheel wells aren't in the same place. They list the bed width at the wall as 60" for the 1500 and 66" for the 2500. They don't mention the width of the top rails, though.
